In order to achieve the dual conditions of high output and low fuel consumption in automobile engines, a lean combustion technique is known wherein the air-fuel ratio (hereinafter referred to as AFR) is changed to provide leaner AFR than the theoretical AFR value corresponding to the engine running conditions.
In Tokkai Sho 60-45742 published by the Japanese Patent Office, an AFR controller is disclosed wherein the AFR is changed to a leaner AFR than the theoretical AFR when, for example, the cooling water temperature is greater than 80.degree. C., the throttle valve opening is no greater than a predetermined value, and the rate of change of the speed of the vehicle is no greater than a predetermined value.
In this AFR controller, however, the fuel quantity supplied to the engine fluctuates sharply when the AFR is changed over, and the torque produced by the engine fluctuates widely. A torque shock therefore occurs, and passengers in the vehicle experience an uncomfortable jolt.